Design of Graded Transition Joints Through Thermodynamic and Kinetic Modeling
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DISSIMILAR metal welds (DMWs) are commonly used in energy applications where varying material behavior is necessary due to the high-temperature operating conditions. (Acronyms used in this article are summarized in Table I.) DMWs are typically produced by joining a ferritic [body-centered cubic matrix (bcc)] steel such 2.25Cr1Mo (Grade 22) to alloy 800H with an austenitic [face centered cubic (FCC)] matrix using a nickel-base filler metal. The ferritic steel is used in lower temperature, less corrosive environments, whereas the austenitic alloy is used in higher temperature regions where enhanced creep strength and
JONATHAN P. GALLER and JOHN N. DUPONT are with Lehigh University, 5 East Packer Avenue, Bethlehem PA 18015. Contact e-mail: [email protected] SUDARSANAM SURESH BABU and MOHAN SUBRAMANIAN are with the University of Tennessee, Knoxville, 1512 Middle Drive, Knoxville TN 37996. Manuscript submitted May 30, 2018. Article published online March 18, 2019 METALLURGICAL AND MATERIALS TRANSACTIONS A
corrosion resistance are required. In this application, the alloys are usually joined using standard fusion welding processes. This produces very abrupt and uncontrolled gradients in composition, microstructure, and resultant thermal and mechanical properties.[1,2] These sharp gradients lead to premature failure in the ferritic steel near the weld interface, which is a primary concern of the power generation industry. These failures can result in forced plant outages that may cost a power company up to $850,000 per day in lost revenue and repair costs.[3] A large body of work has been conducted to study the mechanisms that lead to premature failure,[1,4–8] and the failure mechanism has recently been reviewed in detail.[2] The failures can be attributed to three major factors. The first is the difference in carbon concentration between the two materials, and the microstructural evolution due to carbon diffusion during aging. This promotes sharp changes in microstructure and concomitant mechanical properties along the fusion line. The second factor is the formation of carbides along the fusion line in the heat-affected zone (HAZ) of the ferritic material due to carbon migration during service. A
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Table I. Description of Acronyms Dissimilar Metal Weld Body-Centered Cubic Face-Centered Cubic Heat-Affected Zone Coefficient of Thermal Expansion Chemical Potential of Carbon Partially Mixed Zone Graded Transition Joint Energy-Dispersive Spectroscopy Optical Emission Spectroscopy Niobium Carbide Chromium Carbide
DMW BCC FCC HAZ CTE CPC PMZ GTJ EDS OES NbC M23C6
typical DMW failure is initiated from creep voids that form at the carbide-matrix interface.[6,9] Therefore, the presence of these carbides creates a microstructure that is susceptible to creep failure. Finally, the coefficient of thermal expansion (CTE) mismatch between the ferritic and austenitic materials can cause strain localization along the weld interface. This, combined with the previously mentioned issues, exacerbates the susceptibility to f
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